A series of established mouse cell lines which have acquired a transformed, tumor-like phenotype after exposure to chemical carcinogens have been found to behave in a fashion consistent with "dominant" expression of the transformed phenotype. We are in the process of characterizing the genetic basis by isolation of the gene (Tf+) and its normal counterpart, using gene transfer andDNA techniques. We have recently extended these approaches to a cell line in culture derived from a human tumor (retinoblastoma).